via comedy central
A man walks into a brothel and says "I only have five bucks - is there anything you can do for me?"
The Madam says "Yes", and gives him a duck.
The man hands over the cash, and disappears upstairs with the duck.
A couple of weeks later he comes back. "I have ten bucks this time." he says, "What'll that get me?"
The madam hands the man another duck.
"Hold on!" says the man, "How come this duck is ten when the other one was only five? "They're both ducks!"
"Ah yes, that's right", says the Madam, "But this one doesn't have syphillis."